1. Field of the Invention
The present invention relates generally to control of audio signal gain, and more specifically, to audio signal gain control techniques that modulate between discrete gain values.
2. Background of the Invention
Audio gain control circuits are included in nearly every audio device, for example gain control circuits provide volume control in consumer audio devices such as MP3 players and car and home audio systems, as well as in professional audio devices. Present-day audio gain control is typically performed in steps according to a digital control value, with the size of the steps determining the resolution of the gain adjustment. In analog audio gain control circuits having discrete gain steps selected by a digital control value, e.g., when a switchable resistor ladder is used to set a feedback or input resistor value, or when an otherwise continuous gain control element is controlled with a control signal having discrete steps, a change in gain is audible as a pop when the input signal has significant DC offset and/or the offset changes with the change in gain. The pop will occur even when no audio is present. If an audio signal is present and the gain change is made at any time other than a zero-crossing of the audio signal, a step will also be introduced in the audio signal, also causing an audible pop.
When such a gain control is changed continuously through a number of steps, the result is a sequence of repetitive pops, an artifact that is sometimes referred to as “zipper” noise, due to its resemblance to the sound of operation of a zipper. The repetition rate of the pops is generally below the audible band and therefore heard as clicks, rather than an audible tone.
An offset in the audio signal may be due to a DC-coupled input and offset that is already present in an input signal, or the offset may be produced by the input bias current(s) of a gain stage interacting with the mismatch in resistance at the input node(s). The amplifier may also have a gain-dependent offset. For example, if the amplifier has an output-referenced offset of 10 mV at 0 dB of gain, the offset will jump to 20 mV when the gain is changed to 6 dB, and a “pop” will be produced. In particular, when discrete gain control is provided by switching resistors connected to the input node of an operational amplifier, a variable voltage offset that depends on the gain selection can be expected at the output of the operational amplifier, even when no DC offset is present in the input signal. If DC offset is present in the input signal, then a signal step equal to the gain change multiplied by the signal offset plus any change in offset due to changes in the gain setting resistances will occur when the gain changes, assuming that the AC portion of the audio signal has zero level when the gain changes.
When any of the above-described circuits are used as a volume control or in another type of gain control stage, such as in an audio signal processing codec integrated circuit, zipper noise results when the gain level is changed rapidly between multiple levels. A pop will occur if the gain is changed once, or multiple pops will occur if the gain is changed slowly. In order to mitigate the effects of such gain changes, in applications where the gain change can be synchronized with the signal-zero crossings or when the signal can be muted during gain transitions, large resistors are used to reduce the offset changes, and a large number of discrete gain settings (and therefore a corresponding large number of resistors) are needed to reduce the effect of the pops.
Therefore, it would be desirable to provide an audio gain control circuit in which signal step changes due to gain adjustments do not produce large audible artifacts. It would further be desirable to provide an audio gain control circuit where the number of discrete gain settings can be reduced while maintaining a fine degree of gain control.